Chromogranin A (CGA) is a prohormone, abundant in endocrine secretory vesicles and ubiquitous in brain, that plays a role in attenuating the secretion of parathyroid hormone (PTH). Our working hypothesis is that CGA, which is cosecreted from the parathyroid gland with PTH in response to lowered extracellular calcium concentration, can be processed to amino terminal peptides, which in turn inhibit PTH secretion. This function would thus affect the deposition/resorption of bone, as well as calcium-dependent intracellular processes. A synthetic 40-residue peptide containing a disulfide bond between residues 17 and 38 is fully active in the inhibition of parathyroid secretion. Using modified synthetic peptides, we have identified several structural features which are critical for biological activity. In order to fully understand the relationship of structure to function, a three dimensional structure of the peptide is required. We propose to solve the three dimensional structure by standard homonuclear techniques.